Pea crab

Pea crab, any member of a genus (Pinnotheres) of crabs (order Decapoda) living in the mantle cavity of certain bivalve mollusks, echinoderms, and polychaetes as a commensal (i.e., on or in another animal host but not deriving nourishment from it). Females of Pinnotheres ostreum, also known as the oyster crab, are found in oysters of the Atlantic coastal waters of North America and are especially abundant in oysters of Chesapeake Bay. The body of the female is pinkish white and up to 2 cm (about 0.75 inch) across. An irregular stripe runs from front to back across the carapace, or back. Males, seldom seen, are smaller, dark brown, and usually free-swimming. The female pea crab holds her eggs with the back legs until they hatch. The larvae leave their molluscan home and swim before settling in the mantle cavity of another bivalve.

Pea crab (Pinnotheres pisum) in opened shell

Jane Burton/Bruce Coleman Ltd.

P. maculatus, with a range similar to that of P. ostreum, is found in the shells of scallops, clams, and mussels. P. pisum, found in European coastal waters, lives in mussel and cockle shells.

Learn More in these related articles:

Though no crab is truly parasitic, some live commensally with other animals. One example is the little pea crab (Pinnotheridae), which lives within the shells of mussels and a variety of other mollusks, worm-tubes, and echinoderms and shares its hosts’ food; another example is the coral-gall crab (Hapalocarcinidae), which irritates the growing tips of certain corals so that they grow to enclose...